Despite the wide variety of species that have complex social structures – elephants, monkeys, chimps, dolphins, giraffes, wolves, corvids, and lots more – many have argued that jealousy requires a sophisticated understanding of the self and of other's social goals and desires.

That skepticism has proven reasonable in the case of guilt. What many dog owners report as guilt is probably the dog's learned response to the owner's own scolding behavior. As I wrote in 2012 at Scientific American, dogs probably give the "guilty look" because they've learned that it reduces the likelihood or severity of their scolding, not because they know they've broken a rule.

But dog owners may actually be right when it comes to jealousy. That's because young infants and toddlers, with their immature, developing brains, appear capable of at least simple forms of the emotion. Indeed, it was after reading a small but growing literature on jealousy in babies that Christine R. Harris and Caroline Prouvost of the University of California, San Diego, decided to adapt the experiment for pet dogs.

In the baby experiments, six-month-old infants showed jealous reactions when their mothers interacted with another infant (actually a lifelike doll), but not when their mothers interacted with a non-social object (a book). That suggested to the researchers that "jealousy may have a primordial or core form that can be triggered without complex cognition about the self or about the meaning of the social interaction." The primordial jealousy response can be provoked, they explain, by the "relatively simple perception that an attachment figure or loved one's attention has been captured by a potential usurper." The baby then does what it can to regain the attention of their loved one. More complex forms of jealousy experienced by human adults could build on simpler forms that are present in human infants.

If that's the case, then other social species might also be capable of demonstrating primordial jealousy. The basic structure of jealousy – whether primordial or complex – is straightforward: it emerges from a social triangle. When an interloper threatens an existing, important relationship, the outcome can be jealousy. That's the case for sexual or romantic relationships threatened by real or imagined infidelity, or for parent-child relationships threatened by siblings. Jealousy can also arise from non-kin relationships. The same underlying process that leads to jealousy in sexual or familial relationships can apply to any friendship of social or emotional value.

Harris and Prouvost simply adapted the baby experiment for dogs. Thirty-six dogs and their owners participated in their experiment. For safety purposes, the experiment was restricted to dogs under 35 pounds and shorter than 15 inches ("because of the possibility that the jealousy manipulation would result in aggression and small dogs could be more easily controlled in such circumstances"). Owners, who weren't aware of the true purpose of the experiment, interacted with a realistic looking stuffed dog that barked and wagged its tail, and as well as with two objects – a Halloween candy pail, and a book – while their dogs looked on.

The owners were instructed to interact affectionately with the pail, just as they had with the stuffed dog. If the dogs responded with jealousy to the stuffed dog but not the candy pail, that would suggest that the dogs could attenuate their jealousy, directing it at other dogs in particular. The fact that most of the dogs sniffed the doll's anal region suggests that they indeed perceived it as real. If the dogs responded jealously even when their owners read from the book, that would suggest that it wasn't jealousy at all that motivated their so-called "jealous behavior," but instead a simple lack of attention.

The researchers found that the dogs were much more likely to display jealousy behaviors, including aggression – snapping, getting between the owner and the object, and pushing the object away – for the stuffed dog compared with either the candy pail or the book. Not that the dogs wouldn't rather have all of their owners' attention on themselves: when engaging with the candy pail, the dogs still spent more time looking at their owners than for the book. While the dogs weren't thrilled to be ignored in favor of the candy pail, they directed their jealousy appropriately, just at the canine interloper.

Harris and Prouvost conclude the primordial form of jealousy isn't unique to human infants, but is shared at least with domestic dogs, if not with other animals. "This emotional state does not presuppose complex interpretations of the behavior of the rival and the attachment figure and its meaning to the self," they say, although "such cognitions clearly can impact jealousy in adult interpersonal relationships."

While primordial jealousy emerges early in human development and may have emerged early in evolution, that doesn't mean that the subjective emotional experience is equivalent to the type of jealousy that human adults experience.

Did jealousy evolve out of sexual or familial relationships? Could it have emerged in animals that require group cooperation for survival, in which alliances between individuals are malleable and are therefore open to threats from outsiders?

Since dogs (and wolves) are born into litters, have pair bonding, and hunt cooperatively, they won't be particularly useful for better understanding the evolutionary origins of jealousy. Other species could prove more informative: domestic cats, for example, bear litters but do not pair bond.

It is also possible that it was domestication itself that gave rise to dogs' capacity for even simple forms of jealousy. It would be prudent to see whether wolves also have primordial jealousy. And even if they do, dogs could be unique in their capacity for cross-species jealousy. Given how attuned they are to human social cues, research with horses could shed light on that question.

As with most interesting research, this finding raises more questions than it provides answers. It is clear, at least, that jealousy is not a uniquely human emotion. But where does it come from? Are jealous behaviors actually effective at regaining the attention of a social partner? Are dogs alone in participating in multi-species social triangles, or is jealousy another marker of domestication? If nothing else, this study is yet another reminder that our species is not alone in experiencing social emotions.